Abstract: An insert is disclosed for use with a sliding panel door having a sash and rollers for supporting the door on a rail of a supporting frame. The insert is adapted to be secured to the sash in a vertically adjustable position and extend longitudinally between the rollers. When the sliding door is mounted on the rail, a portion of the insert surrounds the head of the rail and prevents the door from becoming dislodged from the rail by lateral forces. The insert also provides security against the sliding door being lifted and removed from the rail by an intruder. Furthermore, a weather strip can be secured to the insert to reduce air and water leakage under the door.